This invention is concerned with bentonite clay and water-soluble aluminum salt compositions which are useful in replacing the binding clay in foundry sand molds which has been deactivated by molten metal poured into the mold.
In foundry operations, the sand molds used to form the metal products cast therein may contain a clay material such as benonite clay, either alone or also containing carbonaceous additives, which acts as a binder for the sand. After casting hot metal in the sand mold, a portion of the clay contained therein is deactivated along with some additives and must be replaced. To replace the deactivated material, an aqueous slurry of fresh bentonite clay is prepared, with approriate additives as required, and pumped to a muller,wherein it is combined with foundry sand and mulled. This sand and clay mull is then rammed into a mold. Presently, the clay content of the slurries used to replace clay in sand molds is from about 8% to 11% by weight. At less than about 8%, the gel strengths of the slurries are too low to hold the additives in suspension. At greater than 11% by weight the viscosities of the slurries are so high that they can hardly be pumped by conventional equipement. However, with foundries pouring a large quantity of metal into the sand mold, clay quantities greater than those existing in the normal slurry make-up are required. Thus there is a need for slurries containing greater than 11% by weight of clay which have viscosities low enough to allow pumping. In addition, since control of water is critical in a foundry sand mix, the more clay which can be incorporated into the slurry, the better the foundry can regulate the moisture content in the sand without having to consider the loss or over-addition of clay.
It is known in the art that certain colloidal sodium and potassium aluminum silicates such as bentonite clay undergo swelling of the clay lattice structure by polar molecules such as water. This phenomenon is discussed more fully in the treatise, "Clay Minerology", 7th Edition by R. Grim, (McGraw-Hill). Water, by virture of its doplar character, according to Grim, will orient itself first positive to negative with respect to the charged surfaces of the clay lattice and then with itself, forming layers of a crystalline-like substance similar to the mechanism of forming ice crystals. This mechanism accounts for the ability of such clays to thicken polar media into which they are incorporated. A depression or interruption of this arrangement will serve to consequently lower the viscosity of the media.
It has been found that water-soluble aluminum salts such as aluminum sulfate disrupts the formation of the crystlline-type water arrangement that causes the clay to expand as the water lattice grows. This disruption decreases the thickness of the absorbed water layer in the clay molecule and thus the clay will not swell as much as normal when hydrated.
Consequently the viscosities and gel strengths of aqueous media containing the clay and aluminum salt are reduced. Therefore high concentrations of clay slurries, e.g., greater than 11% by weight, can be prepared and easily pumped by conventional equipment for foundry applications. Foundries employing such clay slurries for make-up purposes can also regulate moisture more easily without undue concern over clay additions to the sand. Moreover, the aluminum salt in the clay does not substantially affect the binding properties of the clay. Other cations, such as calcium and magnesium cation on the other hand are high enough in activity to replace sodium if in sufficient concentration, consequently adversly affecting the clay binding properties. Additions of small amounts of water to a sand mix containing a bentonite clay and water-soluble aluminum salt will dilute the aluminum salt and the sand mix will retain its original properties.
In the case of a calcium compound on the other hand, although a decrease in the swelling of the clay is achieved, calcium ion will exchange with sodium in the clay creating a calcium bentonite which will not redisperse on dilution and the mix will experience a change in physical properties.
The present invention makes use of the properties of water-soluble aluminum salts as viscosity depressants for bentonite clays by providing compositions of clay and water-soluble aluminum salts useful in making pumpable slurries for mulling with foundry sands which are used to replace the deactivated clay in sand molds. The slurries can contain greater than 11% by weight of clay and up to about 26% by weight and yet have sufficient low viscosities to be easily pumped and have sufficient gel strengths to maintain solids in dispersion.